1. Field of the Invention
The present invention relates to a fire retardant tablet produced by tableting a mixture containing a fire retardant as the essential component, a method of fire retardation of a resin employing the tableted fire retardant, and a fire-retardant polymer composition comprising the fire-retardant resin. In particular, the present invention relates to a fire-retardant polymer composition having excellent fire retardancy, and a molded article therefrom.
2. Description of the Related Art
Heat-expandable graphite is known to exhibit excellent fire retardancy in combination with one or more of fire retardant auxiliaries including phosphorus compounds such as red phosphorus and ammonium polyphosphate; metal oxides such as antimony trioxide and zinc borate; and metal hydroxides such as aluminum hydroxide and magnesium hydroxide.
The fire-retardant polymer compositions, which contain the heat-expandable graphite and have excellent fire retardancy, are usually produced by mixing a resin with heat-expandable graphite and the above fire retardant synergist; mixing and kneading in a kneading machine such as a single screw extruder or a twin screw extruder to produce a fire retardant compound or a fire retardant master batch; and then molding the fire retardant compound by a molding machine such as an injection machine, or mixing the fire retardant master batch with a resin and molding the mixture by a molding machine such as an injection machine.
The fire retardant master batch produced by melt-blending and containing the heat-expandable graphite is limited in the maximum fire retardant concentration to be about 50%, and the content of the resin component (hereinafter referred to as a "binder") is higher. Therefore, the binder affects significantly the mechanical properties of a molded article produced from a resin and the master batch. This disadvantageously necessitates production of master batches for respective kinds of resins.
The heat-expandable graphite, in blending with the resin in production of the fire retardant compound or master batch, comes to be crushed by melt-shearing into smaller size particles, resulting in lower fire retardancy.
To prevent the crushing of the heat-expandable graphite in the blending with the resin in production of the fire-retardant polymer composition, the kneading is conducted at an extremely low shearing rate with a single screw extruder, or the blending is conducted with a twin screw extruder by feeding the heat expandable graphite to a side-feeder of the extruder. However, the former method does not give high productivity and does not give sufficient fire retardancy owing to poor dispersion of the heat-expandable graphite. The latter method prevents the crushing of the heat-expandable graphite to some extent, but not sufficiently.
JP-A-6-25476, JP-A-6-25485, JP-A-6-73251, and so forth disclose methods of achieving the higher fire retardancy by using the heat-expandable graphite having an expansion property in the c-axis direction in rapid heating, and a limited particle size on a 80 mesh sieve. These methods, however, cannot compensate for the decrease of the fire retardancy caused by crushing of the heat-expandable graphite in kneading with the resin.
The present invention is made to solve the above problems.